1. Field of the Invention
The present invention generally relates to a contact system for a current limiting unit, such a current limiting unit having a molded housing and further including: a movable wiper with a moving contact tip on one end and an axis of rotation on the other end; a supply lead to be connected to the wiper, and which supply lead makes a transition into a first current lead; a stationary contact carrier with a stationary contact tip on one end and a second current lead on the other end, preferably with a partial segment with a current direction opposite to that in the wiper; and contact compression springs mounted on one end of the wiper.
The present invention can also particularly relate to a contact system for current limiting units which are combined with power circuit breakers or motor circuit breakers.
2. Background Information
European Patent Application No. 418 755 A2 discloses a three-phase current limiting unit with such a contact system, as shown in FIG. 2 of that application. Three current-limiting contact systems are next to one another in a molded housing. Each contact system essentially consists of a stationary contact carrier with a stationary contact tip and a movable wiper with a movable contact tip. The contact carrier and wiper run parallel to one another over a significant portion of their length. Each contact carrier makes a transition into a load-side connection terminal lug. Each wiper is mounted on an axis of rotation and is connected by means of a flexible connecting conductor (flexible lead) to a supply lead which is to be connected to the corresponding output terminal of a power circuit breaker. Each wiper is acted on by a helical rotation spring, or compression spring, attached to the axis of rotation, so that the moving contact tip is connected under a corresponding contact pressure to the stationary contact tip. In the event of a short circuit, the electrodynamic repulsive forces which are generated by the short circuit current running in opposite directions in the contact carrier, and in the wiper, exceed the contact pressure force, which leads to the spinning off of the wiper and thus to the opening of the contact system. The arc formed between the separated contact tips is deionized in an arc extinction chamber.
European Patent Application No. 418 755 A2 also discloses an additional current-limiting contact system, as shown in FIG. 24 of that application. In this contact system, the wiper is mounted on a bearing block connected to the base of the molded housing by means of its axis of rotation. Between the wiper and the bearing block, helical tension springs are engaged on both sides by means of interlocking dowels, to generate the necessary contact pressure.
The disadvantages of these contact systems are that the contact systems, as a result of the necessary high prestress of the contact compression springs, can only generally be mounted by means of a complex and time-consuming process, and a great deal of force is usually required to install them in the molded housing. Furthermore, the opening angle during the spinning-off of the wiper is essentially not defined, and the current transmission essentially does not take place directly between the wiper and the supply lead. The spinning-off tends to be limited either by the counter forces of the contact compression springs or by stops on the molded housing. In the former case, the contact compression springs could be damaged by excessive stretching, and in the latter case parts of the molded housing could be damaged by impacts or melting. As a result of the force of the interposed flexible connecting conductor, the electrodynamic behavior and thus the lower limit for the opening short circuit current cannot typically be reproduced with sufficient accuracy.
German Patent Application No. 34 11 276 C2 discloses a power circuit breaker with a contact system whose wiper can pivot, and is connected in an electrically conductive manner to a bearing, or journal, on which journal the axis, axle or shaft of rotation for the fork-shaped end of the wiper is mounted. Prestressing means essentially guarantee a sufficiently conductive connection between the wiper and the bearing, which bearing makes a transition within a single piece into a terminal bracket. For its part, the terminal bracket is connected to a current connection with the interposition of a bimetallic heating element.
The solution disclosed in the above-cited German Patent Application does not appear to represent a pure current limiter, and is apparently designed to reverse the electrodynamically opened wiper on next breaker mechanism to be tripped. The motion of the spun-off wiper is disadvantageously limited by the moving parts connected to the actuating shaft. This known contact system is also rather difficult to assemble and install, since on one hand a very considerable number of individual parts, some of which are very complex, must apparently be assembled, and on the other hand, during the assembly process, it appears to be necessary to overcome the strong spring forces of the contact compression springs engaged between the wiper and the axis of rotation.